1. Field of the Invention
The present invention relates, generally, to two-speed (high-low) transfer cases and, more particularly, to a method and apparatus for providing synchronized shifts between low and high gearing arrangements in a digitally controlled two-speed transfer case in a motor vehicle employing a Controller Area Network (CAN) digital data bus system.
2. Description of the Related Art
Transfer cases are used in full and part-time, four-wheel drive vehicles to distribute driving power received through an input shaft from the vehicle""s transmission to a pair of output drive shafts. One of the drive shafts powers the vehicle""s front wheels and the other of the drive shafts powers the vehicle""s rear wheels. In vehicles that permit shifting between two-wheel drive and four-wheel drive modes, the input shaft of the transfer case provides continuous drive power to one of its output shafts and selectively provides drive power to the other output shaft via some type of disengageable or otherwise adjustable coupling, such as a viscous coupling, electromagnetic clutch, or positionable spur gearing. Other drive modes are sometimes provided, including four-wheel drive high (4H) for higher four-wheel drive speeds, four-wheel drive low (4L) for lower four-wheel drive speeds, neutral for disengaging the transmission from the front and rear axles to allow towing, and locked four-wheel drive for controlling wheel slippage. Historically, transfer cases were configured such that the vehicle had to be stopped before shifting between low and high gears. Typically, this requirement resulted from the lack of any type of synchronizer within the transfer case to facilitate this shift. In these cases, an adjustable coupling was used to manually shift between drive modes using a mechanical shift actuator.
On the other hand, synchronization of the input and output shafts of the transfer case prior to shifting between low and high gears and into and out of four-wheel drive facilitates shifts while the vehicle is moving. This mode of operation has been referred to as shift xe2x80x9con the fly.xe2x80x9d Numerous synchronization devices have been proposed in the related art. For example, it is known to employ a clutch which is operable to translate torque either directly between the input and output shafts (high speed) or between the input and output shafts via a planetary gear reduction set. Clutches of this type may be spring-biased such that shifting is not fully accomplished until the relative speeds of the rotating members to be coupled have achieved a certain level of synchronization. However, some synchronization devices known on the related art have suffered from the disadvantage that they are overly complex and add excessive cost to the transfer case. Others have failed to achieve adequate synchronization prior to the shifting event resulting in slip and/or unacceptable noise.
Mechanical shift actuators gave way to electronically controlled shift actuators, particularly for shift actuators that can be operated by a rotational source, such as an electric motor. U.S. Pat. No. 4,664,217 issued to Welsh et al. on May 12, 1987 discloses such an electric shift actuator. More specifically, the Welsh et al. ""217 patent teaches the use of a reversible DC electric motor to rotate a cammed shift actuator to selectively shift drive gearing within the transfer case between a neutral position, two-wheel drive mode and low and high speed four-wheel drive modes. Selection of a desired drive mode is accomplished by operating the motor under the control of a microprocessor-based control circuit. The microprocessor commands a motor drive circuit to energize the motor to run in either the clockwise or counterclockwise direction to achieve the desired drive mode. While this type of electronic shift control was an improvement in the related art, the problem of effectively and efficiently synchronizing the rotation of members to be coupled during any given shifting operation remained.
U.S. Pat. No. 5,771,477 issued to Showalter et al. on Jun. 23, 1998, proposed one solution for this problem. More specifically, the Showalter ""477 patent discloses a method and apparatus for synchronizing low to high transfer case shifts using sensors to sense the speed of the input and output shafts of the transfer case. A microprocessor is employed to measure the change in speed over time of each of the input and output shafts and to make a prediction when the relative speeds of the shafts will be equal. The microprocessor then commands operation of a shift actuator at a predetermined time before the shafts are synchronized such that shifting is accomplished when the speeds of the input and output shafts are substantially equal. While the ""477 method and apparatus was an improvement over the related art, there still remains a need for greater control and more accurate synchronization prior to shifting between low and high speeds in a transfer case.
Correspondingly, as transfer case art developed, the complexity of the vehicles in which transfer cases are used has also evolved, revealing further shortcomings in the present state of transfer case design. One manner in which vehicle complexity has increased is the notable design trend toward integrated vehicle systems and controls. This had lead to a progression of innovations in interconnected computer controlled vehicle systems, with each succeeding model year moving closer to complete computer and electronic control of the vehicle. The shortcomings in the present state of transfer case design have become apparent as the progression toward complete vehicle system and sub-systems integration has found its way to four-wheel drive vehicle platforms. Specifically, in addition to the need to provide improved synchronized shifting for economy and efficiency, transfer cases also need to be incorporated into the overall vehicle system control interface as well.
In moving toward total electronic control of vehicle systems, manufactures have had to cope with the expanding complexity in all automotive systems and sub-systems in general. As the vehicle systems themselves have become more complex and interconnected the number of individually dedicated point-to-point wire connections between systems, controllers, and sensors has dramatically increased. This has translated to larger, heavier and more cumbersome wiring harnesses, which must carry varying voltages and currents, are difficult to manufacture and install, and are susceptible to mechanical and environmental stresses. Additionally, the larger, more sophisticated harnesses add to the production and maintenance costs of the vehicle. In response to this problem, vehicle manufactures have begun to replace the discrete, dedicated wiring with common digital interfaces or data bus networks. This type of digital system interface is known as a small area network (SAN).
In application, these SANs are simple digital wiring systems, also known as a digital data bus, similar to computer network systems. The SAN is routed through the vehicle and replaces a large percentage of the costly and bulky discrete, point-to-point wiring. One specific type of SAN being used in automobiles is the Controller Area Network (CAN) system. The CAN system is a proven, pre-existing, international SAN standard that has been adopted by some vehicle manufactures. It is a readily available, off-the-shelf system that utilizes a minimum of additional components within the vehicle. The CAN system electronically interconnects all the network members by a simple two wire, twisted pair cable and provides high-speed serial digital data transfer between all members in the system. The network members consist of the various vehicle systems and sub-systems, or in many cases, their electronic control units.
In operation, one of the CAN system members may be an on-board computer or microprocessor serving as a vehicle management system responsible for the overall control of the vehicle. The vehicle""s management system communicates with the various system controllers (i.e., engine, fuel control, braking, suspension, climate control, etc.) through the CAN system while deriving discrete inputs and feedback from the vehicle operator and various sensors about the vehicle. The CAN system may also be designed without a centralized microprocessor, allowing direct interface between all the system controllers through the digital data bus cable. As such, the CAN system offers accurate high-speed digital interface between all the vehicle""s systems and sub-systems. Additionally, since the vehicles systems are interconnected, constant system-wide monitoring for detailed maintenance and troubleshooting data collection is available.
In this manner, the CAN system greatly increases the efficiency and cost-effectiveness of design, production, and operation of a vehicle. However, this highlights a current disadvantage of contemporary transfer case design, which is the fact that they have not been adapted to work with a digital data bus system such as the CAN system. Current transfer case designs lack the ability to function with the CAN system so that a contemporary transfer case would be isolated from the rest of the vehicle""s computer controlled and monitored systems. Since current transfer cases suffer from this lack of electronic control and digital interface with the other vehicle systems, they are not able to provide the truly coordinated and synchronized LO to HI shifting that is desired.
Therefore, considering the shortcomings of the prior art transfer cases in that they are designed for conventionally wired vehicles, cannot interface to a digital data system such as the CAN system, and have limited synchronization methods, there is a need for a transfer case that can be digitally controlled and a method to provide fully synchronized shifting of a digitally controlled transfer case in a vehicle that employs a CAN digital data network system.
The present invention overcomes the disadvantages in conventional transfer cases by providing a method to digitally control a transfer case through a digital data bus to provide synchronized low to high shift capabilities in vehicles that employ a controller area network (CAN) system. More specifically, a method is provided comprising the first step of initiating an actuator to move a clutch from engagement with a gear set to a predetermined neutral synchronization point. The method also includes the step of sensing the speed of the actuator and routing the signal to the control unit. The time for the clutch to reach the predetermined neutral synchronization point as a function of the sensed speed of the actuator and the known remaining rotational distance through which the actuator must move for the clutch to reach the predetermined neutral synchronization point is then determined used this method. The method also includes the step of maintaining the actuation of the clutch until it has reached the predetermined neutral synchronization point. The actuator is then commanded to stop when the calculated time has elapsed for the clutch to reach the predetermined neutral synchronization point. The rotational speed of the transfer case input and output shafts is then sensed and compared to determining a difference value. The method further includes the steps of commanding changes in engine RPM and automatic transmission gear settings through the control area network to bring the difference value within a predetermined range and completing the shift from low to high by commanding further actuation of the clutch from the predetermined neutral synchronization point to a direct torque engagement of the input and output shafts when the calculated difference in the rotational speeds of the input and output shafts is determined to fall within a set predetermined range, indicating synchronization of the shafts.
The present invention also overcomes the disadvantages in conventional transfer cases by providing a transfer case that can be digitally controlled which provides synchronized low to high shifts and is adapted to be integrated into a vehicle with a controller area network (CAN) system. More specifically, the transfer case includes an input shaft, an output shaft, and a reduction gear set selectively operable to translate torque between the input and output shafts at reduced speeds. A clutch is operable to translate torque between the input and output shafts either directly or through the reduction gear set. An actuator, having a rotational output, is operatively coupled to the clutch for moving the clutch between predetermined positions. A control unit is employed in connection with the transfer case to affect the method of synchronizing low to high shifts in the transfer case. To this end, the control unit operatively moves the clutch from engagement with the gear reduction set to a neutral position. The control unit senses the speed of the actuator as it moves the clutch to the neutral position. The control unit further determines the time until the clutch will reach a predetermined neutral synchronization point using the sensed speed of the actuator and the remaining known rotational distance through which the actuator must move for the clutch to reach the predetermined neutral synchronization point. The control unit is further operable to stop the actuator when the clutch has reached the neutral synchronization point. The control unit then senses and compares the rotational speeds of the input and output shafts and signals the actuator to complete the shift from the neutral synchronization point to high by moving the clutch to provide direct torque translation between the input and output shafts when the difference in the rotational speeds of the input and output shafts is less than a predetermined value. The control unit further has a digital data interface to allow the transfer of digital data between the control unit and other vehicle systems through the CAN system.
Accordingly, the present invention is a method and an apparatus for providing synchronization of low to high shifts in a transfer case that is digitally controlled within a vehicle utilizing the CAN system, allowing for a high level of control and an accurate synchronization during the shift between low and high speeds. Furthermore, these objectives are achieved with the method and apparatus of the present invention in an efficient, cost effective and relatively simple manner.
Other objects, features and advantages of the present invention will be readily appreciated, as the same becomes better understood after reading the subsequent description taken in connection with the accompanying drawings.